This application is based on and incorporates herein by reference Japanese Patent Application No. 2000-28193 filed Feb. 4, 2000.
The present invention relates to an ultrasonic sonar for detecting the presence of an obstacle by transmitting an ultrasonic wave and receiving a wave reflected from the obstacle and for calculating the distance to the obstacle.
Ultrasonic sonars used for vehicles are known as back sonars and as corner sonars. Such a vehicular sonar generally comprises an ultrasonic transceiver (transmitter and receiver using an ultrasonic oscillator) mounted in a bumper of a vehicle. This transceiver transmits pulses of an ultrasonic wave and receives the echo pulses from an obstacle. The travel time of the ultrasonic wave is measured, and the distance to the obstacle is calculated from the measured time.
More specifically, the envelope of the echo wave is detected. If the detected envelope exceeds a certain threshold level, it is determined that there is an obstacle. Then, the distance to the obstacle is calculated from the duration between the instant when the ultrasonic wave is transmitted and the instant when the threshold level is exceeded. The sonar informs the driver of the presence of an obstacle according to the result of the calculation of the distance. That is, when the vehicle parks or makes a turn, the possibility of contact with an obstacle is detected. Then, the sonar warns a vehicle driver to avoid possible collision with the obstacle.
The ultrasonic sonar comprises a single ultrasonic oscillator that transmits and receives an ultrasonic wave. Even after the ultrasonic wave transmitting operation of the ultrasonic oscillator is stopped, the oscillator continues to oscillate due to its mechanical inertia, that is, reverberation. Where both transmission and reception are performed by one ultrasonic oscillator by a pulse radar system and the envelope of an echo wave is detected to detect an obstacle, the echo wave cannot be distinguished from the reverberation if the echo wave is received before the reverberation disappears. Consequently, it is difficult to detect obstacles which exists in proximity, for instance, in less than about 25 cm.
In Japanese Patent Laid-Open No. 268035/1998, it is proposed that an oscillation frequency f1 different from the resonant frequency fr of a transceiver (ultrasonic sensor) is used. Detection is made with a filter that passes the frequency of |frxe2x88x92f1|, which corresponds to a beat frequency. Where an echo wave is superimposed on reverberation, if the signal is amplified, the portion of the output from the amplifier that indicates the echo wave (reception) saturates due to excessive amplification. Specifically, the gain must be so set that even weak echo waves from distant obstacles can be detected. Therefore, saturation occurs if an echo wave from an obstacle in close proximity is received. Hence, the beat frequency |frxe2x88x92f1|does not appear and thus the received signal cannot be detected.
Accordingly, it is an object of the present invention to provide an ultrasonic sonar capable of detecting obstacles in proximity.
It is another object of the invention to provide a method of transmitting an ultrasonic wave from an ultrasonic sonar.
According to one aspect of the present invention, an ultrasonic oscillator transmits an ultrasonic wave. The ultrasonic wave having a frequency different from the reverberation frequency is transmitted, because the reverberation frequency is determined by the ultrasonic oscillator and the circuit constant of an associated electronic circuit and does not depend on the transmission frequency. An echo wave reflected by an obstacle is received by the ultrasonic oscillator. The signal received by the ultrasonic oscillator is converted from analog to digital form. The digital signal undergoes digital signal processing to extract only the transmission frequency component to detect the obstacle. That is, the reflected wave included in the reverberation is extracted from the received wave by digital signal processing.
According to another aspect of the present invention, an ultrasonic wave is transmitted with a given frequency. The reverberation frequency is calculated. A frequency different from the calculated reverberation frequency is set and stored in a memory. An ultrasonic wave having the frequency stored in the memory is transmitted from the ultrasonic oscillator. As a result, it is made possible to make it less susceptible to variations among individual commercial products available as sonars, that is, variations in reverberation among individual commercial products.